The present invention relates generally to the art of high-voltage circuit breakers and particularly to circuit breakers with gas-insulated tanks enclosing separable contacts.
Gas-insulated circuit breakers are known. Such circuit breakers generally include separable contact elements contained within a sealed tank filled with an inert insulating gas for reducing arcing. Specifically, the gas is typically sulfurhexaflouride (SF.sub.6) due to its good insulative and arc interruption properties.
Despite careful manufacturing practices, an assembled circuit breaker tank can contain undesirable foreign particles, such as dust, metal shavings from machined parts, etc. Although unlikely, it is also possible that hardware, e.g., nuts, washers, screws, etc., could work loose during operation. It is undesirable for such particles, particularly metallic conductive or semiconductive particles, to freely reside within the tank. Such particles, if permitted to remain free, can interfere with the operation of the circuit breaker, causing undue arcing, flashing or promoting breakdown between metallic components. The presence of particles greatly reduces the breakdown voltage of the circuit breaker. Sensitivity to particles increases with the voltage across the circuit breaker due to the increased electric field stress levels. Circuit breakers are now constructed capable of handling very high voltages, for example 362 kV and higher.
In a conventional tank, the operation of the contacts can cause such particles to move about. For example, it is known that the operation of opening and closing the contacts causes shocks and vibrations capable of moving loose particles within the tank. Also, in a "puffer" type circuit breaker, the operation of opening the contacts results in flows of SF.sub.6 gas capable of blowing loose particles around the tank.
Traps for foreign particles are known in the art of high-voltage gas insulated transmission lines, but it has been previously believed for several reasons that a particle trap in a tank section of a high voltage circuit breaker would be ineffective. A primary reason is that circuit breakers are not field tested with a "conditioning" AC voltage as are gas insulated lines. The use of particle traps in transmission lines requires that an AC test voltage be gradually raised from low levels to a level higher than the operating voltage. This moves particles into receptacles provided in the gas chamber designed to trap the particles before the particles can cause a breakdown. Furthermore, the severe mechanical shock that occurs in a circuit breaker and the blasts of SF.sub.6 gas in "puffer" contacts could force undesirable movement of particles.
Efforts have been made to include a particle filter or trap in gas insulated systems which include circuit breakers, however these systems utilize means remote from the circuit breaker tank, such as within the aforementioned gas insulated transmission lines, to attempt to capture particles present in gas flows through the system. U.S. Pat. No. 4,219,366 discloses a gas-insulated system including circuit breakers wherein gas insulated transmission lines connected to the circuit breakers include a sticky coating. U.S. Pat. No. 3,943,315 discloses a particle filter located in a circuit of SF.sub.6 flow pipes supplying SF.sub.6 gas to a circuit breaker tank from an external SF.sub.6 gas source and compressor.
It is desirable to immobilize foreign particles within a tank of a high-voltage circuit breaker in order to assure effective circuit breaker operation and to minimize potential damage.